1. Field of the Invention
The present invention relates to an apparatus for driving a photoconductive liquid crystal light valve, which is used in a projection type liquid crystal display (LCD) apparatus and is coupled to light writing means, and also, to a display apparatus using a photoconductive liquid crystal light valve.
2. Description of the Background Information
FIG. 1 shows an example of a projection type LCD apparatus that comprises a display photoconductive liquid crystal light valve.
Referring to this diagram, a drive voltage signal output from a drive voltage source 101 is applied across two electrodes (to be described later) of a photoconductive liquid crystal light valve 1. Means for writing an image on the liquid crystal light valve 1 may be, for example, an optical fiber light tube, which comprises a CRT (Cathode Ray Tube) 3 to which a video signal V.sub.V is supplied via a video amplifier 2, and an optical fiber 4, coupled to the CRT 3, to guide an image displayed on the CRT to the liquid crystal light valve 1. The output image of the CRT 3 is written on the photoconductive film of the liquid crystal light valve 1. The video amplifier 2 is capable of changing the level of the output video signal in accordance with a control signal V.sub.c to thereby alter the output image of the CRT 3 or the level of the writing light to the liquid crystal light valve 1.
Reading light, emitted from-a light source 5, will be incident via a mirror 6 and through a condenser lens 7 upon a polarization beam splitter 8. The incident light has a P polarization component and an S polarization component. The former component directly passes through the polarization beam splitter 8, while the latter has its direction turned by 90 degrees to enter the liquid crystal light valve 1.
If an image drawn is on the liquid crystal layer of the liquid crystal light valve 1, the light reflected by the liquid crystal light valve 1 locally includes a P polarization component according to the tint of the image on the liquid crystal layer. This P polarization component in the reflected light alone directly passes through the polarization beam splitter 8 to enter a projection lens 9, so that the image corresponding to that P polarization component is projected on a screen 10.
FIG. 2 illustrates the structure of the photoconductive liquid crystal light valve 1 used in the projection type LCD apparatus.
In this diagram, a spacer 12 is placed around a liquid crystal layer 11, with alignment films 13 and 14 provided on the respective sides of the layer 11. Sandwiched between the liquid crystal layer 11 and a photoconductive film 15, which may be formed of amorphous silicon, are a dielectric mirror 16 as a light reflecting film and a light shielding film 17. The light reflecting film 16 serves to reflect projection light coming from the reading side, while the light shielding film 17 serves to absorb leak light from the light reflecting film 16 to block the light to the photoconductive film 15. Transparent conductive films 18 and 19, serving as two electrodes, are provided outside the liquid crystal layer 11 and the photoconductive layer 15 respectively to sandwich the liquid crystal layer 11. All the components mentioned above are sealed by glass substrates 20 and 21.
When an image is drawn on the photoconductive film 15 in the thus constituted photoconductive liquid crystal light valve 1 by the writing light incident from the right-hand side (writing side) in FIG. 2, the internal resistance of the photoconductive film 15 changes locally according to the tint of the image. The drive voltage across the transparent electrodes 18 and 19 is applied to the liquid crystal layer 11 adjacent to those portions which correspond to the resistance-changed portions, in accordance with the tint of the image, causing spatial modulation.
Referring again to FIG. 1, before the liquid crystal light valve 1, CRT 3 and optical fiber 4 are coupled together, the output to input characteristic of the CRT 3 is adjusted to a specific characteristic. This adjustment is done to make the light writing characteristic to the liquid crystal light valve 1 constant to acquire a stable and good projection image. This is actually accomplished by arranging a photometer to the front surface or the display surface of the CRT 3 before the liquid crystal light valve 1 is coupled to the CRT 3 by the optical fiber 4, measuring the level of the output light of the CRT 3 by means of photoelectric transfer of the photometer when a video signal V.sub.V of predetermined level and pattern is supplied to the CRT 3, and supplying a control signal V.sub.c of the level that should provide a predetermined level of output light to the video amplifier 2 based on the result of the measurement.
The video amplifier 2 includes an offset controller 201 a gain controller 202, and a drive amplifier 203 as shown in FIG. 3. The offset and gain for amplifying the input video signal V.sub.V are controlled by control signals V.sub.c1 and V.sub.c2, respectively. The levels of those control signals may be changed, for example, by a user manually altering volume controls Vol1 and Vol2 on the operation section (not shown). The solid line io in FIG. 4 represents the initial characteristic of the CRT 3 that is to be controlled in the above manner to satisfy the predetermined output light level characteristic. In the diagram, the horizontal scale V.sub.cg [V] is the voltage that is applied between the cathode and grid of the CRT 3 according to the input video signal V.sub.V, and the vertical scale I [W/cm.sup.2 ] is the level of the output light or the output power of the CRT 3 that is produced according to that voltage V.sub.cg. For the illustrated initial characteristic, it is apparent that the proper white level I.sub.w and black level I.sub.B are acquired by supplying such a video signal as to apply the voltages V.sub.cgWO and V.sub.cgB0 to the CRT 3.
The liquid crystal light valve 1 is coupled to the CRT 3 after the output writing light to input video signal characteristic of the CRT 3 to the liquid crystal light valve 1 are adjusted. The coupling should be made with a high precision, which is inevitable to provide a high-quality image.
If the light writing characteristic varies due to some time series changes by a change in heater current that drives the CRT 3 or a change in fluorescent body due to a temperature rise on the tube surface, after the liquid crystal light valve 1, CRT 3 and optical fiber 4 are coupled together, the characteristic as represented by the broken line ia in FIG. 4 will be provided. In this case, the black level and white level of the CRT 3 change from those of the initial characteristic, degrading the gray scale characteristic and shifting the white balance. To cope with them requires a troublesome work to detach the CRT 3 in close contact with the liquid crystal light valve 1, detect the light writing characteristic using the photometer, readjust the characteristic, and then couple the CRT 3 again to the liquid crystal light valve 1.
Further, since the user manually controls the light writing characteristic of the CRT, not only the image quality will vary in each product, but also the prompt and accurate adjustment cannot be expected.